# std::function<R(Args...)>::function

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< cpp‎ | utility‎ | functional‎ | function

function() noexcept; | (1) | (since C++11)
function( std::nullptr_t ) noexcept; | (2) | (since C++11)
function( const function& other ); | (3) | (since C++11)
| (4) |
function( function&& other ); |  | (since C++11) (until C++20)
function( function&& other ) noexcept; |  | (since C++20)
template< class F > function( F&& f ); | (5) | (since C++11)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc ) noexcept; | (6) | (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, std::nullptr_t ) noexcept; | (7) | (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, const function& other ); | (8) | (since C++11) (removed in C++17)
template< class Alloc > function( std::allocator_arg_t, const Alloc& alloc, function&& other ); | (9) | (since C++11) (removed in C++17)
template< class F, class Alloc > function( std::allocator_arg_t, const Alloc& alloc, F f ); | (10) | (since C++11) (removed in C++17)

Constructs a `std::function` from a variety of sources.

1,2) Creates an empty `std::function`.
3) Copies the target of other to the target of *this.
If other is empty, *this will be empty right after the call too.
4) Moves the target of other to the target of *this.
If other is empty, *this will be empty right after the call too.
other is in a valid but unspecified state right after the call.
5) Initializes the target with std::forward<F>(f). The target is of type std::decay<F>::type.
If f is a null pointer to function, a null pointer to member, or an empty value of some `std::function` specialization, *this will be empty right after the call.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_same_v<std::remove_cvref_t<F>, std::function<R(Args...)> is false. | (since C++23)

- An lvalue of type std::decay<F>::type is callable for argument types `Args...` and return type `R`.

If std::is_copy_constructible_v<std::decay_t<F>> or std::is_constructible_v<std::decay_t<F>, F> is false, the program is ill-formed. | (since C++23)

If `F` is not CopyConstructible, the behavior is undefined.
6-10) Same as (1-5) except that alloc is used to allocate memory for any internal data structures that the `std::function` might use.

When the target is a function pointer or a std::reference_wrapper, small object optimization is guaranteed, that is, these targets are always directly stored inside the std::function object, no dynamic allocation takes place. Other large objects may be constructed in dynamic allocated storage and accessed by the std::function object through a pointer.

### Parameters

other | - | the function object used to initialize *this
f | - | a callable object used to initialize *this
alloc | - | an Allocator used for internal memory allocation
Type requirements
- `Alloc` must meet the requirements of Allocator.

### Exceptions

3,8,9) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object.

4) Does not throw if other's target is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the constructor used to copy or move the stored callable object. | (until C++20)

5,10) Does not throw if f is a function pointer or a std::reference_wrapper, otherwise may throw std::bad_alloc or any exception thrown by the copy constructor of the stored callable object.

### Notes

`std::function`'s allocator support was poorly specified and inconsistently implemented. Some implementations do not provide overloads (6-10) at all, some provide the overloads but ignore the supplied allocator argument, and some provide the overloads and use the supplied allocator for construction but not when the `std::function` is reassigned. As a result, allocator support was removed in C++17.

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <utility>

    void print_num(int i) { std::cout << "print_num(" << i << ")\n"; }

    int main()
    {
        std::function<void(int)> func1; // (1) empty constructor
        try
        {
            func1(333 << 1);
        }
        catch (const std::bad_function_call& ex)
        {
            std::cout << "1) " << ex.what() << '\n';
        }

        std::function<void(int)> func2{nullptr}; // (2) empty constructor
        try
        {
            func1(222 * 3);
        }
        catch (const std::bad_function_call& ex)
        {
            std::cout << "2) " << ex.what() << '\n';
        }

        func1 = print_num; // initializes func1 using assignment operator

        std::function<void(int)> func3{func1}; // (3) copy constructor
        func3(33);

        std::function<void(int)> func4{std::move(func3)}; // (4) move constructor,
                                                          // func3 in unspecified state
        func4(44);

        std::function<void(int)> func5{print_num}; // (5) constructor with function
        func5(55);

        // (5) constructor with lambda
        std::function<void(int)> func6([](int i) { std::cout << "lambda(" << i << ")\n"; });
        func6(66);
    }

Possible output:

    1) bad_function_call
    2) bad_function_call
    print_num(33)
    print_num(44)
    print_num(55)
    lambda(66)

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2132 | C++11 | overloads (5,10) might be ambiguous | constrained
LWG 2774 | C++11 | (5,10) performed an additional move | eliminated

### See also

(constructor) | constructs a new `std::move_only_function` object (public member function of `std::move_only_function`)

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